Isobutene can be used to produce a number of desired chemicals. For example, etherification processes make high octane compounds which are used as blending components in lead-free gasoline. These etherification processes will usually produce ethers by combination of an isoolefin with a monohydroxyl alcohol such as methanol or ethanol. The etherification process can also be used as a means to produce pure isoolefins by cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C4 isoolefins as the feedstock. Apart from the production of MTBE, isobutene can also be used to form other chemicals, including isoprene and isooctane, to name a few.
Typically, a normal butane feed is selectively isomerized to produce isobutane which can be dehydrogenated to form isobutene. However, many dehydrogenation catalysts are particularly sensitive to normal butene. Normal butene can further dehydrogenate forming butadiene which is highly prone to coking, thus normal butenes must be separated from any stream entering the dehydrogenation zone.
Accordingly, most complexes include a fractionation column, such as a deisobutanizer column having about 120 or more distillation trays that is require to separate normal butane and butenes from isobutane. With respect to providing a recycle isobutane feed, the effluent stream produced by the dehydrogenation zone typically contains a low concentration of normal C4 hydrocarbons, due to isomerization activity in the dehydrogenation zone. Therefore, the unconverted C4 hydrocarbons are typically returned to the deisobutanizer column to separate out the normal C4 hydrocarbons.
Furthermore, olefins and dienes in the unconverted stream are typically fully statured to remove any olefins returning to the deisobutanizer column—since normal butene can be harmful for the dehydrogenation zone and olefins can be harmful for the isomerization zone. Typically the saturation zone also requires an oxygenate removal zone to remove any oxygenates from entering the saturation zone to protect the catalyst in the saturation zone.
While these processes are presumable effective for their intended purposes, it is believed that modified process flow schemes may provide for more efficient and economical separation of the isobutane recycle stream.
Moreover, some complexes are provided with a high purity isobutane feed stock, and thus, do not have an isomerization section and a fractionation column. Therefore, the unconverted iC4 hydrocarbons do not have the option to return to the deisobutanizer column. Accordingly, the unconverted iC4 hydrocarbons are typically treated in a raffinate column. However, a raffinate column having about 60 distillation trays typically does not have the ability to separate normal hydrocarbons from iso hydrocarbons as well as a deisobutanizer column can.
Therefore, there remains a need for effective and efficient processes for providing a recycle isobutane feed that does not include normal butane and butenes. It is believed to be desirable in some aspects to eliminate the saturation zone and the required oxygenate removal zone. It is also believed to be desirable in some aspects to optimize fractionation to reduce cost.